The present invention relates to an arrangement for controlling a hydraulic motor. More particularly it relates to an arrangement for controlling a hydraulic motor with a throttling directional valve, a speed lowering brake valve controlling a discharge side of the hydraulic motor and having a control piston with a control side and a port connected with a port of a directional valve, and a control valve arranged between the control side of the control piston and a supply side of the hydraulic motor.
Arrangements of the above-mentioned general type are known in the art. In a known arrangement, a lowering speed brake valve is directly controlled by a pump pressure which is adjusted between a pump and the hydraulic motor. With varying pump pressure, the control piston of the speed lowering brake valve correspondingly acts upon these variations and the valve member of the speed lowering brake valve correspondingly controls the variations. Thereby different pressure takes place between the directional valve port connected with the speed lowering brake valve and the direction valve port connected with the supply container. As a result of this, with the same position of the valve member of the directional valve, different pressure medium streams flow from the hydraulic motor to the supply container (German magazine "Fluid", February 1979, pages 31-33).
For controlling the hydraulic motor independently from its loading, it is known to connect a pressure regulating valve between the control side of the speed lowering brake valve and the hydraulic motor so that the control conduit of the pressure regulating valve is connected with a conduit connected with the speed lowering brake valve, and a chamber accommodating a pressure spring is connected with the brake valve port connected with the directional valve. Thereby the pressure at the brake valve port connected with the direction valve is held constant, whereby the directional valve port connected with the speed lowering brake valve and the supply container is held constant. Pressure variations at the pump or consumer side no longer act upon the flowing back liquid quantity. The speed lowering brake valve acts as a pressure manometer, whereby its braking function is not affected. However, the constant pressure difference at the directional valve acts at the side of the high pressure in addition to the differently applied spring force of the pressure spring during the displacement of the control piston and the different forces due to the flow at the control piston acted upon with constant control pressure. Since the control piston is always located in condition of force equilibrium when the sum of the three above-mentioned forces is equal to the force from the constant control pressure, the equilibrium is established with a very great force from the flow, before the high pressure of the pressure differential maintains constant at a directional valve obtains its nominal value (German Auslegeschrift No. 2,911,891).